Avant sometimes stutters on YouTube, Facebook and sometimes many other sites. Usually the stuttering is most noticeable on HDDs. SSDs can alleviate the problems to some degree. The stuttering also continues well after the user has closed the tabs, and can re-appear if they close and open the browser again.

I believe this happens because the session in memory becomes large, and the Firefox renderer writes to disk every 15 seconds to ensure the session is not forgotten in case of a crash. This writing doesn't increase CPU usage, instead it causes a temporary deadlock in the kernel which appears to us as a lag spike.
Avant already stores the same information in its own session file and thus this Firefox feature does not benefit us at all, instead it causes lag and stutter.

I've increased this from 15 seconds to 1500 seconds, this makes it far less noticeable especially during watching movies.

Step 1
In the browser URL bar, type in:about:config
and hit enter, you may have to click past a disclaimer.

Step 2
in the new search bar that appeared, type in:browser.sessionstore

Step 3
In the filtered list, you can find a line called:browser.sessionstore.interval
Double click on this and change the value from 15000 to 1500000

Restart your browser and youre done!
No more intermittent lag!

Maybe Avant should be released pre-tweaked with this variable set higher.